1. Field of the Invention
The present invention relates to a phase modulation apparatus for modulating a carrier frequency signal using an inputted digital baseband modulation signal by carrying out modulation using a PLL (Phase Locked Loop), and wireless communication apparatus using this phase modulation apparatus.
2. Description of the Related Art
In the related art, a phase modulation apparatus employing PLL where a carrier signal is modulated by a baseband modulation signal so as to form a transmission signal (namely, a baseband modulation signal is up-converted to a wireless frequency) is widely employed. This type of phase modulation apparatus generally requires low cost, low power consumption, superior noise characteristics and high transmission characteristics, for example, precise modulation. Various configurations, such as a configuration where a modulation signal is inputted to a frequency divider or a configuration where a modulation signal is inputted to a VCO, are possible as a configuration for the phase modulation apparatus using this PLL. Further, a two-point modulation method is also proposed where modulation inside a PLL waveband and modulation outside a PLL waveband are carried out at two different locations (VCO and frequency divider) (see, for example, Japanese Patent Application Tokuhyo 2003-510899).
A configuration for a phase modulation apparatus using a broadband modulation PLL adopting this proposed two-point modulation method is shown in FIG. 22. As shown in the same drawing, this phase modulation apparatus is equipped with a PLL containing reference oscillator 1, limiter 2, reference frequency divider 3, phase frequency detector 4, charge pump 5, loop filter 6, voltage-controlled oscillator (VCO) 8, frequency divider 9 and adder 7, modulator 10, digital sigma modulator 13, charge pump scaling 16, modulation scaling 17, adders 11, 14, constant F12 and constant P15.
Voltage-controlled oscillator 8 of the PLL outputs an RF modulation signal. The oscillation frequency of this RF modulation signal changes according to a voltage inputted to control voltage terminal Vt of VCO 8. Frequency divider 9 divides the frequency of an RF modulation signal outputted by voltage controlled oscillator 8.
Phase frequency detector 4 compares the phase of a signal outputted by frequency divider 9 and the phase of a reference signal from reference oscillator 1, and outputs a signal (current) corresponding to the phase difference of both signals. Charge pump 5 converts the output current of phase frequency detector 4 to a voltage to output to loop filter 6. Control of residual modulation within the phase-locked loop is implemented as a result of charge pump 5 being controlled by charge pump scaling 16, and as a result, more accurate two-point modulation is possible. Loop filter 6 averages the output signal of charge pump 5.
Modulation scaling 17 is capable of scaling a modulation signal based on modulation data (Ka), namely capable of controlling amplitude scale of a modulation signal. Scaling at this modulation scaling 17 is carried out so as to ensure that, even if VCO sensitivity fluctuations (changing of the gradient of FIG. 23) occur as a result of manufacturing variation and temperature fluctuation, the amplitude scale of the output of the VCO before and after fluctuation is kept constant, and deterioration of this modulation precision is prevented as a result.
Multimode phase modulation from a narrow band to a broad band is therefore implemented using the two-point modulation method as in the above.
Further, even in technology of the related art shown in Japanese Patent Application Laid-Open No. 2004-7704, a voltage gain stage is provided where sensitivity fluctuations of a VCO due to manufacturing variations and temperature fluctuations are kept constant, and a configuration is adopted where frequency deviation is then drawn out from an error signal and a phase amplitude amount is controlled. Therefore, deterioration of modulation precision due to VCO sensitivity fluctuation is prevented.
As shown in FIG. 23, a region indicating non-linear characteristic exists between the input voltage and the output frequency at the VCO as shown in FIG. 23. When a PLL is locked at the region indicating a linear characteristic as shown in FIG. 23A, there is no deterioration in modulation precision. However, when the PLL is locked in the non-linear region or in the vicinity of the non-linear region, as shown in FIG. 23B, the input voltage is applied to a non-linear region which results in a problem that deterioration in a transmission characteristic, for example, deterioration in modulation precision.
In particular, when modulation scaling is carried out according to VCO sensitivity as shown in the technology of the related art, scaling is carried out so that amplitude increases when VCO sensitivity falls, but the possibility becomes still higher where VCO input voltage is applied to non-linear regions as a result. By such modulation scaling, deterioration of transmission characteristics such as, for example, modulation precision caused by application of a VCO input voltage to a non-linear region cannot be prevented.
Further, normally, the VCO input signal is an analog signal, and a D/A converter (provided at the latter stage of the modulation scale when modulation scaling is carried out according to VCO sensitivity fluctuations as in the related art) is therefore necessary. This D/A converter outputs an analog signal of an amplitude corresponding to an input bit string. A large peak appears at this analog signal. In this case, when precision of phase modulation is made high, it is necessary to adjust the number of bits of the input bit string of the D/A converter at the peak portion and it is therefore necessary for the number of bits to be large. When the number of bits is increased, a problem occurs where the circuit scale and the power consumption increase.